The present invention relates generally to microwave test equipment, and more particularly, to an automated noise measurement test system for making phase noise and amplitude noise measurements of microwave signals using waveguide, coax and fiber optic delay lines discriminators.
Two conventional approaches for making phase noise measurements are primarily employed by the RF community. The first is a noise measurement test system that uses a waveguide delay line discriminator, and the second uses a test system that measures the combined noise from two phase-locked RF sources; one source is a low noise reference oscillator and the other source is the (oscillator) unit under test (UUT). There are also several less common types of test systems, including cavity-discriminator and three-oscillator mixing. These conventional test systems do not have the combined sensitivity capability and flexibility in a rugged, compact, and/or affordable unit to measure new, low phase noise microwave sources that are currently under development and current low noise sources. In addition, the conventional test systems are generally relatively large, costly and inflexible.
It has been found that, by modifying the currently used waveguide-based noise measurement test system, it is possible to provide a test system with performance that exceeds the capability of commercially available systems and currently used military waveguide-based noise measurement systems. The two oscillator mixing approach requires one ultra-low noise source to evaluate low noise sources and this can result in considerable additional expense since the second ultra-low noise oscillator must be provided. The major disadvantage of the current waveguide-based test system is its large size, weight, inflexibility and comparative cost. Also, no surrently available systems are fully automated, which means that a skilled operator and appreciable setup time is required to use the systems.
Several techniques employed in the present invention, referred to as an automated noise measurement test system, are generally described in articles, and a patent for a fiber optic phase noise test set, U.S. Pat. No. 4,918,373 assigned to an assignee of the present invention. The articles include "New Discriminator Boosts Phase-Noise Testing", in Microwaves, March 1982, "Extending the Range and Accuracy of Phase Noise Measurements", 42nd Annual Frequency Control Symposium, 1988, "The Measurement of Noise in Microwave Transmitters", IEEE Transactions on Microwave Theory & Techniques, Vol. MTT-25 No. 4, April 1977, "Cross-Correlation Phase Noise Measurements", 1992 IEEE Frequency Control Symposium, and "Using Digital Data Processing to Speed Up Radar Phase Noise Measurements", Autotestcon, September 1994, pp. 205-210. However, while the present invention has applied some of the techniques disclosed in these different publications, the techniques have been refined and incorporated into a modular, flexible, compact and affordable test system that is not disclosed in the references.
Accordingly, it is an objective of the present invention to provide for a noise measurement test system for making phase noise and amplitude noise measurements of microwave signals that overcomes the limitations of conventional systems.